For example, in the digital recording/playback of audio, video, or the like, original audio, video, or the like are reproduced by recording a digital signal (a series of sampled and quantized levels) in a predetermined recording medium such as semiconductor memory, and an optical disk through A/D conversion of analog signals of the audio, video, or the like, and performing the D/A conversion of the digital signal after reading the digital signal from the recording medium when playing back. If it is desired to save the capacity of the recording medium, it can be performed to shorten quantization bit length when performing the A/D conversion of the analog signals, but quantization noise arises in the digital signal read from the recording medium. For this reason, interpolation processing is performed by not only expanding the quantization bit length of the digital signal, read from the recording medium, by the predetermined bit length toward a low-order side, but also inputting the digital signal to a variable low pass filter whose frequency characteristic (cutoff frequency) dynamically changes according to an input waveform as shown in FIG. 22.
For example, if an original signal is a sinusoidal analog signal as shown in FIG. 23(1), a digital signal after A/D conversion is as shown in FIG. 23(2), but a digital signal the interpolation processing of which is performed by being inputted into a variable low pass filter after bit expansion is as shown in FIG. 23(3), the digital signal is near to the original sinusoidal wave, and hence, it can be seen that quantization noise is considerably improved.